CS268: Beyond TCP Congestion Control Kevin Lai February 4, 2003.
-
Upload
tobias-reynolds -
Category
Documents
-
view
213 -
download
0
Transcript of CS268: Beyond TCP Congestion Control Kevin Lai February 4, 2003.
CS268: Beyond TCP Congestion Control
Kevin Lai
February 4, 2003
2
TCP Problems
When TCP congestion control was originally designed in 1988:
- Maximum link bandwidth: 10Mb/s
- Users were mostly from academic and government organizations (i.e., well-behaved)
- Almost all links were wired (i.e., negligible error rate)
Thus, current problems with TCP:- High bandwidth-delay product paths
- Selfish users
- Wireless (or any high error links)
3
High Bandwidth-Delay Product Paths
Motivation- 10Gb/s links now common in Internet core
• as a result of Wave Division Multiplexing (WDM), link bandwidth 2x/9 months
- Some users have access to and need for 10Gb/s end-to-end
• e.g., very large scientific, financial databases
- Satellite/Interplanetary links have a high delay Problems
- slow start
- Additive increase, multiplicative decrease (AIMD) Congestion Control for High Bandwidth-Delay Product Networks. Dina Katabi, Mark
Handley, and Charlie Rohrs. Proceedings on ACM Sigcomm 2002.
4
Slow Start
TCP throughput controlled by congestion window (cwnd) size
In slow start, window increases exponentially, but may not be enough
example: 10Gb/s, 200ms RTT, 1460B payload, assume no loss
- Time to fill pipe: 18 round trips = 3.6 seconds- Data transferred until then: 382MB- Throughput at that time: 382MB / 3.6s = 850Mb/s- 8.5% utilization not very good
Loose only one packet drop out of slow start into AIMD (even worse)
5
AIMD
In AIMD, cwnd increases by 1 packet/ RTT Available bandwidth could be large
- e.g., 2 flows share a 10Gb/s link, one flow finishes available bandwidth is 5Gb/s
- e.g., suffer loss during slow start drop into AIMD at probably much less than 10Gb/s
time to reach 100% utilization is proportional to available bandwidth
- e.g., 5Gb/s available, 200ms RTT, 1460B payload 17,000s
6
Simulation Results
Round Trip Delay (sec)
Avg
. T
CP
Util
iza t
ion
Bottleneck Bandwidth (Mb/s)
Avg
. T
CP
Util
iza t
ion
Shown analytically in [Low01] and via simulations
50 flows in both directionsBuffer = BW x Delay
RTT = 80 ms
50 flows in both directionsBuffer = BW x Delay
BW = 155 Mb/s
7
Proposed Solution:
Decouple Congestion Control from Fairness
Example: In TCP, Additive-Increase Multiplicative-Decrease (AIMD) controls both
Coupled because a single mechanism controls both
How does decoupling solve the problem?
1. To control congestion: use MIMD which shows fast response
2. To control fairness: use AIMD which converges to fairness
8
Characteristics of Solution
1. Improved Congestion Control (in high bandwidth-delay & conventional environments):
• Small queues
• Almost no drops
2. Improved Fairness
3. Scalable (no per-flow state)
4. Flexible bandwidth allocation: min-max fairness, proportional fairness, differential bandwidth allocation,…
9
XCP: An eXplicit Control Protocol
1. Congestion Controller2. Fairness Controller
10
Feedback
Round Trip Time
Congestion Window
Congestion Header
Feedback
Round Trip Time
Congestion Window
How does XCP Work?
Feedback = + 0.1 packet
11
Feedback = + 0.1 packet
Round Trip Time
Congestion Window
Feedback = - 0.3 packet
How does XCP Work?
12
Congestion Window = Congestion Window + Feedback
Routers compute feedback without any per-flow state
Routers compute feedback without any per-flow state
How does XCP Work?
XCP extends ECN and CSFQ
13
How Does an XCP Router Compute the Feedback?
Congestion Controller Fairness ControllerGoal: Divides between flows to converge to fairness
Looks at a flow’s state in Congestion Header
Algorithm:
If > 0 Divide equally between flows
If < 0 Divide between flows proportionally to their current rates
MIMD AIMD
Goal: Matches input traffic to link capacity & drains the queue
Looks at aggregate traffic & queue
Algorithm:
Aggregate traffic changes by ~ Spare Bandwidth ~ - Queue Size
So, = davg Spare - Queue
14
= davg Spare - Queue
224
0 2 and
Theorem: System converges to optimal utilization (i.e., stable) for any link bandwidth, delay, number of sources if:
(Proof based on Nyquist Criterion)
Details
Congestion Controller Fairness Controller
No Parameter Tuning
No Parameter Tuning
Algorithm:
If > 0 Divide equally between flows
If < 0 Divide between flows proportionally to their current rates
Need to estimate number of flows N
Tinpkts pktpkt RTTCwndTN
)/(1
RTTpkt : Round Trip Time in header
Cwndpkt : Congestion Window in header
T: Counting Interval
No Per-Flow State
No Per-Flow State
15
BottleneckS1
S2
R1, R2, …, Rn
Sn
Subset of Results
Similar behavior over:
16
Av g
. U
tiliz
a tio
n
Av g
. U
tiliz
a tio
n
XCP Remains Efficient as Bandwidth or Delay Increases
Bottleneck Bandwidth (Mb/s) Round Trip Delay (sec)
Utilization as a function of Delay
XCP increases proportionally to spare bandwidth
and chosen to make XCP robust to delay
Utilization as a function of Bandwidth
17
XCP Shows Faster Response than TCP
XCP shows fast response!XCP shows fast response!
Start 40 Flows
Start 40 Flows
Stop the 40 Flows
Stop the 40 Flows
18
XCP is Fairer than TCP
Flow ID
Different RTTSame RTT
Avg
. Th
rou
gh
pu
t
Flow ID
Avg
. Th
rou
gh
pu
t
(RTT is 40 ms 330 ms )
19
XCP Summary
XCP - Outperforms TCP
- Efficient for any bandwidth
- Efficient for any delay
- Scalable (no per flow state)
Benefits of Decoupling- Use MIMD for congestion control which can grab/release
large bandwidth quickly
- Use AIMD for fairness which converges to fair bandwidth allocation
20
Selfish Users
Motivation- Many users would sacrifice overall system efficiency for more
performance- Even more users would sacrifice fairness for more
performance- Users can modify their TCP stacks so that they can receive
data from a normal server at an un-congestion controlled rate. Problem
- How to prevent users from doing this?- General problem: How to design protocols that deal with lack
of trust? TCP Congestion Control with a Misbehaving Receiver. Stefan Savage, Neal
Cardwell, David Wetherall and Tom Anderson. ACM Computer Communications Review, pp. 71-78, v 29, no 5, October, 1999.
Robust Congestion Signaling. David Wetherall, David Ely, Neil Spring, Stefan Savage and Tom Anderson. IEEE International Conference on Network Protocols, November 2001
21
Ack Division
Receiver sends multiple, distinct acks for the same data
Max: one for each byte in payload
Smart sender can determine this is wrong
22
Optimistic Acking
Receiver acks data it hasn’t received yet
No robust way for sender to detect this on its own
23
Solution: Cumulative Nonce
Sender sends random number (nonce) with each packet
Receiver sends cumulative sum of nonces
if receiver detects loss, it sends back the last nonce it received
Why cumulative?
24
ECN
Explicit Congestion Notification- Router sets bit for congestion
- Receiver should copy bit from packet to ack
- Sender reduces cwnd when it receives ack
Problem: Receiver can clear ECN bit- or increase XCP feedback
Solution: Multiple unmarked packet states- Sender uses multiple unmarked packet states
- Router sets ECN mark, clearing original unmarked state
- Receiver returns packet state in ack
• receiver must guess original state to unmark packet
25
ECN
Receiver must either return ECN bit or guess nonce
More nonce bits less likelihood of cheating
- 1 bit is sufficient
26
Selfish Users Summary
TCP allows selfish users to subvert congestion control
Adding a nonce solves problem efficiently- must modify sender and receiver
Many other protocols not designed with selfish users in mind, allow selfish users to lower overall system efficiency and/or fairness
- e.g., BGP
27
Wireless
Wireless connectivity proliferating- Satellite, line-of-sight microwave, line-of-sight laser,
cellular data (CDMA, GPRS, 3G), wireless LAN (802.11a/b), Bluetooth
- More cell phones than currently allocated IP addresses
Wireless non-congestion related loss- signal fading: distance, buildings, rain, lightning,
microwave ovens, etc.
Non-congestion related loss - reduced efficiency for transport protocols that depend
on loss as implicit congestion signal (e.g. TCP)
28
Problem
0.0E+00
5.0E+05
1.0E+06
1.5E+06
2.0E+06
0 10 20 30 40 50 60
Time (s)
Se
que
nce
nu
mb
er
(byt
es)
TCP Reno(280 Kbps)
Best possible TCP with no errors(1.30 Mbps)
2 MB wide-area TCP transfer over 2 Mbps Lucent WaveLAN (from Hari Balakrishnan)
29
Solutions
Modify transport protocol Modify link layer protocol Hybrid
30
Modify Transport Protocol
Explicit Loss Signal- Distinguish non-congestion losses
- Explicit Loss Notification (ELN) [BK98]
- If packet lost due to interference, set header bit
- Only needs to be deployed at wireless router
- Need to modify end hosts
- How to determine loss cause?
- What if ELN gets lost?
31
Modify Transport Protocol
TCP SACK- TCP sends cumulative ack onlycannot distinguish
multiple losses in a window
- Selective acknowledgement: indicate exactly which packets have not been received
- Allows filling multiple “holes” in window in one RTT
- Quick recovery from a burst of wireless losses
- Still causes TCP to reduce window
32
Modify Link Layer
How does IP convey reliability requirements to link layer?- not all protocols are willing to pay for reliability- Read IP TOS header bits(8)?
• must modify hosts- TCP = 100% reliability, UDP = doesn’t matter?
• what about other degrees?- consequence of lowest common denominator IP architecture
Link layer retransmissions- Wireless link adds seq. numbers and acks below the IP layer- If packet lost, retransmit it- May cause reordering- Causes at least one additional link RTT delay- Some applications need low delay more than reliability e.g. IP
telephony- easy to deploy
33
Modify Link Layer
Forward Error Correction (FEC) codes- k data blocks, use code to generate n>k coded blocks
- can recover original k blocks from any k of the n blocks
- n-k blocks of overhead
- trade bandwidth for loss
- can recover from loss in time independent of link RTT
• useful for links that have long RTT (e.g. satellite)
- pay n-k overhead whether loss or not
• need to adapt n, k depending on current channel conditions
34
Hybrid
Indirect TCP [BB95]- Split TCP connection into two parts- regular TCP from fixed host (FH) to base station- modified TCP from base station to mobile host (MH)- base station fails?- wired path faster than wireless path?
TCP Snoop [BSK95]- Base station snoops TCP packets, infers flow- cache data packets going to wireless side- If dup acks from wireless side, suppress ack and retransmit
from cache- soft state- what about non-TCP protocols?- what if wireless not last hop?
35
Conclusion
Transport protocol modifications not deployed- SACK was deployed because of general utility
Cellular, 802.11b- link level retransmissions
- 802.11b: acks necessary anyway in MAC for collision avoidance
- additional delay is only a few link RTTs (<5ms)
Satellite- FEC because of long RTT issues
Link layer solutions give adequate, predictable performance, easily deployable